The x-radiation generated by the x-ray emitter of an x-ray device and which trans-irradiates the object is not monochromatic, but has an energy spectrum dependent on the tube voltage set. When x-radiation is absorbed during passage through matter, less energetic x-rays are absorbed more strongly than more energetic x-rays because of an energy dependence of the absorption coefficients. This effect, denoted as beam hardening, is substantially dependent on the mean atomic number of the trans-irradiated matter and on the spectral distribution of the x-radiation. This effect is qualitatively higher with increasing atomic number of the matter and with dropping x-ray proton energy.
The object trans-irradiated by the x-radiation is constructed as a rule from various components that partially have quite different absorption coefficients. Moreover, the components are also not distributed in a rotationally symmetrical fashion about a center of rotation of the x-ray device, and so the components are trans-irradiated in different sequence when the projection angles of the recording system are differently set. Because of the beam hardening, no ideal exponential relationship results between the trans-irradiated thickness and signal attenuation in accordance with the attenuation law for monochromatic radiation.
The detector does not acquire the spectral distribution of the x-radiation passing through the object, but only the total energy or the total quantum number of the x-ray photons, and so systematic measured value inconsistencies result that are reflected in typical beam hardening artifacts in the images generated from the measured values. Such inconsistencies can be visualized by artifacts along rays that experience a large hardening effect.
Image artifacts occur, for example, during an examination of a patient's head. In the corresponding tomograms, dark strips are visible in the soft-part tissue between thick bone layers, in particular in the region of the basicranial bone, and these greatly complicate a diagnosis.
Image-correcting measures are necessary for this reason so that the beam hardening artifacts are largely eliminated.
Established methods for computer tomographs for the purpose of eliminating beam hardening artifacts from tomograms of an object having a number of components operate iteratively and require a substantial computational outlay. In a first method step, a temporary tomogram is reconstructed from the measured values of an object that have been obtained from various projection directions. Subsequently, the various components, for example, the bone portions and tissue portions, in the temporary tomogram are identified by way of a segmentation in the pixel image of the layer such that a correction of the measured values can be carried out after reprojection of the component images. Subsequently, there is reconstructed, in turn, from the measured values thus corrected a tomogram in which radiation artifacts continue to be present only in an attenuated form. In order for the beam hardening artifacts to be removed sufficiently well from the tomogram, it is necessary in the case of specific methods to repeat the sequence of segmentation, reprojection, correction and reconstruction of the tomogram until there is a convergence.
By comparison with a simple reconstruction of a tomogram, there is a tripling of the numerical outlay even in the case of only a single iteration step for the known method for correcting beam hardening artifacts, and so it is not always possible to carry out such corrections. The applicability of iterative algorithms for removing beam hardening artifacts becomes all the more difficult, moreover, the more complicated the beam path of the x-radiation through the object. There is also a need for 3D forward projectors or appropriate ray tracers in the field of 3D backprojections, and these are likewise associated with a high numerical outlay.
DE 103 56 116 states a further method for a computer tomograph for reducing beam hardening artifacts. The method includes the acquisition of measured data of an object from various projection directions for two different spectra of the x-radiation, a reconstruction of at least a first and a second temporary energy image, a transformation of the second temporary energy image into a first transformed energy image, and a combination of the first temporary energy image with the first transformed energy image in order to generate a combined first energy image in which beam hardening artifacts are reduced. This method can also be carried out only with a very high numerical outlay.